Many wired and wireless communication systems, e.g. HF, VHF, UHF, radio, etc. operate over communication channels that experience one or more delayed versions of the transmitted waveform at the receiver, a condition known as multipath. Multipath channels degrade the signal quality of the transmitted channel by introducing distortion and/or interference, such as intersymbol interference, in the receiver thereby creating problems with acquiring, demodulating, decoding, and/or accurately extracting the data bits contained in the transmitted signal. This applies to, among other signals, orthogonal frequency division multiplexed (“OFDM”) signals, such as signals defined by U.S. MIL-STD-188-110B Appendix B.
As is known in the art, multipath interference occurs when one radio signal is sent and multiple signals are received at the receiver. The first signal received is the one that travels directly from the transmitter to the receiver, known as the direct path signal. The other signals that are received are delayed versions of the direct path signal and are known in the art as the multipath signals. Typically, the multipath signals may be received a considerable time after the reception of the direct path signal. The multipath signals are typically due to reflection of the transmitted signal by obstacles such as mountains, buildings, trees, weather phenomena, etc. Since the overall path of the reflected signal is longer than the direct path, at the receiver the multipath signals appear to be delayed in time with respect to the direct path signal.
The prior art discloses the use of guard time to overcome the distortion caused by multipath effects. Guard time, as is known in the art, is typically that part of a transmission, for example the first and/or last portions of a communication frame, which is discarded by the receiver as it is assumed to be corrupted by, among other things, multipath interference. The communication frame is therefore divided into a payload portion containing useful data bit energy and a guard time portion which may or may not contain useful data bit energy. Typical prior art receivers do not make use of the energy that is transmitted in the guard time portion of the signal. In some prior art systems, as much as twenty percent of a communication frame is dedicated to guard time which significantly reduces the transmission efficiency of the system.
Since the effects that cause multipath interference are not always constant in time, the amount of guard time required for one communication frame may be different than the amount of guard time required for a different communication frame. However, some typical prior art receivers do not allow for adaptation of the amount of guard time in a communication frame and blindly discard the guard time symbol energy.
Typically, the portion of a frame dedicated to guard time in prior art systems is determined by the largest amount of multipath interference the system is expected to encounter. This amount usually remains constant from frame to frame. For example, if a communication frame 20 msec long is expected to experience a maximum multipath interference of 4 ms, then the first (or last) 4 msec of the communication frame would be designated as guard time and the receiver would not process any energy during that period of time.
Some prior art systems fill the guard time portion of the communication frame with a set of duplicate samples from the payload portion of the frame. This is known as creating guard time by cyclic extension. In these systems, there may be useful energy in the samples in the guard time portion of the communication frame that is not currently being used advantageously by prior art receivers.
Hence, there is a need in the industry for a receiver that is able to determine the amount of multipath interference the communication system is currently experiencing, and advantageously use the signal energy in the guard time portion of the communication frame.
It is therefore an object of the disclosure to provide a novel method for providing data bits from a received signal including frames containing symbols which were transmitted through a channel subject to multipath interference, where the received time domain samples are partitioned into frames; N blocks of received samples are created from a frame; estimated symbols are determined from the received samples; multipath corruption is detected for each block; and for those blocks for which multipath corruption is not detected, the estimated symbols or soft decision bits are coherently combined to thereby provide more reliable data bits.
It is a further object of the disclosure to provide a novel method for processing a received communication frame. For example, in an embodiment, a received communication frame may be processed into N blocks of K samples of which J samples are the guard time portion of the frame where the first of the N blocks contains the first K samples of the frame and the remaining N−1 blocks contain K samples whereby the first sample of each block is offset from the first sample of the previous block by J/N samples.
It is still a further object of the disclosure to provide a novel receiver including means for providing data bits from a received signal including frames containing samples which were transmitted through a channel subject to multipath interference, means for partitioning the received signal into frames; means for creating N blocks of received time domain samples from a frame; means for determining estimated symbols or bit soft decisions from the received blocks; means for determining multipath corruption in the estimated symbols or soft decision bits; means for coherently combining estimated symbols or soft decision bits for those blocks for which multipath corruption is not detected to thereby provide the data bits.
It is yet a further object of the disclosure to provide a novel system for communicating data bits over a channel subject to multipath interference comprising a transmitter for creating communication frames including a payload portion and a guard time portion wherein the samples in the guard time portion are copies of samples in the payload portion, and transmitting the frames; and a receiver for receiving the frames and determining the data bits from estimated symbols or soft decision bits derived from received samples in the payload portion and at least a part of the guard time portion to thereby communicate the data bits.
It is another object of the disclosure to provide a novel system and method for communicating data bits in an orthogonal frequency division multiplexed signal over a channel subject to multipath interference where the receiver creates subsets of the time domain samples in a received frame, determines the estimated symbols or soft decision bits from the received samples, determines which subsets include estimated symbols or bit soft decisions that are corrupted by multipath interference, and coherently adds the energy of the received symbols or bit soft decisions from the subsets which are not corrupted by multipath interference.
These and many other objects and advantages of the present disclosure will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.